[Repo Assist] improve: remove System.Linq from SystemCapability

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🤖 This is an automated PR from Repo Assist.

Removes the using System.Linq; dependency from SystemCapability.cs, replacing three LINQ chains with explicit loops to match the pattern already established throughout the codebase (ExecShellWrapperParser, NodeCapabilities, OpenClawGatewayClient, and others that had LINQ removed in recent PRs).

What changed

ResolveExecutable — PATHEXT extensions list

Before:

extensions.AddRange(pathext.Split(';', StringSplitOptions.RemoveEmptyEntries)
    .Select(e => e.ToLowerInvariant()));

After:

foreach (var e in pathext.Split(';', StringSplitOptions.RemoveEmptyEntries))
    extensions.Add(e.ToLowerInvariant());

Removes the lazy IEnumerable<string> allocation from Enumerable.Select. The List<string> receives elements directly.

HandleExecApprovalsGet — rules summary

Before:

rules = data.Rules.Select(r => new { pattern = ..., ... }).ToArray()

After:

var rulesSummary = new object[rules.Count];
for (var i = 0; i < rules.Count; i++) { ... }
rules = rulesSummary

Pre-allocates the output array to the exact size; avoids the intermediate IEnumerable<> iterator + ToArray() copy.

HandleExecApprovalsSet — shells array parsing

Before:

rule.Shells = shellsEl.EnumerateArray()
    .Where(s => s.ValueKind == JsonValueKind.String)
    .Select(s => s.GetString() ?? "")
    .ToArray();

After:

var shellsList = new List<string>(shellsEl.GetArrayLength());
foreach (var s in shellsEl.EnumerateArray())
{
    if (s.ValueKind == JsonValueKind.String)
        shellsList.Add(s.GetString() ?? "");
}
rule.Shells = shellsList.Count > 0 ? shellsList.ToArray() : null;

Pre-sizes the list from GetArrayLength() (avoids resize), uses a simple guard instead of .Where(), and sets Shells = null for empty arrays (matching the is { Length: > 0 } pattern in ExecApprovalPolicy.Evaluate).

Test Status

• dotnet test OpenClaw.Shared.Tests — 652 passed, 20 skipped (unchanged from baseline)
• No behaviour change; all existing tests pass

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